Superchargers of the type having a turbine driven by engine exhaust gas flow and a compressor driven by the turbine are today widely employed in automotive engines by virtue of several excellent features, for instance, reducing fuel comsumption and suppressing formation of NO.sub.x.
When employing such superchargers in automotive vehicles, it is essential to control the outlet pressure of the compressor or supercharge pressure to an appropriate level by so-called waste gate control, because the engine will knock under excessively high supercharge pressure.
Several types of waste gate controls have been known, one of which is exemplarily disclosed in U.S. Pat. No. 3,035,408 to Alexander Silver et al filed Jan. 4, 1960 and patented May 22, 1962. According to this prior waste gate control, a passage is provided around the turbine for bypassing a portion of the exhaust gases. A by-pass control valve controls exhaust gas flow through the passage in dependence on the ratio of the compressor outlet pressure to the compressor inlet pressure and also on the exhaust gas pressure, so that the turbine driving speed is controlled not to exceed a proper level.
However, such and other waste gate controls are not expedient to reduce the supercharge pressure during engine deceleration. In general, the engine with turbosupercharger has only a poor response to the engine decelerating condition because of its lengthy control cycle. As a vehicle operator abruptly moves the throttle valve to closed position for engine braking effect, the existing high exhaust gas pressure continues to drive the turbine so that the compressor produces yet a high supercharge pressure. It will take a considerable time until the supercharge pressure drops enough for deceration, resulting in a time delay from operator's shifting to deceleration to actual deceleration of the engine. During such delay, the turbine has to do extra work which is not exploited for the engine output power.